1. Field of the Invention
The invention relates to techniques of bending and tempering of glass sheets, particularly to obtain formed safety glazings for motor vehicles.
2. Related Art
Glazings of motor vehicles, with the exception of windshields which most often are of the glass/plastic/glass composite type, are obtained by bending followed by a heat tempering of the glass sheets. The horizontal processes, i.e., those where the glass sheets travel approximately horizontally, are preferred because of the high rates that can be obtained. In these processes, the glass sheet goes through a tunnel furnace while conveyed on a roller bed to reach a forming station.
According to the teaching of French patents FR 2 442 219, FR 2 549 465 and FR 1 476 785, the forming station consists, for example, of a bed of curved rollers or of a guide, so that by advancing the sheet it is gradually deformed until the desired shape is obtained, after which it goes through a tempering zone in which it undergoes an intense cooling by jets of cold air.
Alternatively there are processes using a bending form and in which the glass sheet is applied against the bending form by pneumatic pressure forces (see, for example, European EP 169 770, French FR 2 085 464), then released onto a skeleton, i.e., on a peripheral ring with a curved profile defining the contours of the bent sheet; this skeleton is used as a support for the glass sheet during the later heat tempering operation.
All of these horizontal bending/tempering processes have limits if the radii of curvature of the bent glazings are small, if the bending is about multiple axes (cylindrical or spherical bending) or if the bent glazings exhibit a succession of hills and valleys (saddle type). In these cases, forming by the bed of curved rollers or by pneumatic pressure forces proves incomplete and it is necessary to provide an additional pressing step between a bending form and a counterform, for example by use of a ring (see European EP 143 691 for preforming by a roller bed or French FR 2 085 464, U.S. Pat. No. 4,661,141 for examples of forming finishing by pressing after application of a pneumatic pressure force).
According to the teachings of European EP 143 691, French FR 2 085 465 and U.S. Pat. No. 4,661,141 and others cited above, the pressing ring also supports the glass sheet during tempering. This ring therefore advantageously performs multiple roles of pressing counterform, skeleton for transporting the bent but not yet cooled sheet, and tempering skeleton. These multiple functions impose a somewhat compromised design on the skeleton; actually, to assure tempering of the entire surface of the glass, the air should be able to be evacuated easily on the edges, so the skeleton on which the glass sheet rests should be discontinuous and have numerous ventilation ducts. The drawback of such a discontinuous tempering skeleton is that it produces the risks of marking or optical deformations resulting from the pressing. Further, the more numerous the ventilation ducts (for a more homogeneous tempering of the edges) the smaller the surface on which the glass sheet actually rests and the greater the danger of sliding or even of deformation and marking of the glazing during the phases of acceleration or deceleration during its transport from the bending device to the tempering device. Marking of the glass due to the skeleton can also occur when the skeleton receives the glass sheet which is at its bending temperature (550.degree.-650.degree. C.), whereas the skeleton has just undergone an intense cooling by cold air sent onto the previously supported glass sheet during tempering. Use of material with a low heat transfer coefficient makes it possible to minimize this drawback but this solution considerably increases the cost of the skeletons.
Finally, the skeleton is used during the entire forming phase and especially during the entire lengthy tempering phase. To these two phases are added the times necessary to transfer the skeleton from the bending station to the tempering station, then to the station for recovery of the bent/tempered glass sheet and the return times.
It was also proposed, for example, in European patent application EP 237 231 to operate with two different skeletons: a skeleton used exclusively for pressing and a skeleton for transfer and tempering of the bent glazing. But, besides the fact that it does not solve the problems of thermal shocks and sliding of the glass, this technique requires a new design for the entire glass processing line, including especially a chamber for holding and preheating of the pressing skeleton between two treated sheets.
Further, in French patent FR 993 608, a bending/tempering process is described in which the forming is obtained by molding a glass sheet on a base whose surface has the shape corresponding to the curve to be given to the glass. When the forming is completed, the glass sheet is lifted by auxiliary supports integral with the base to introduce mobile blowing boxes on each side of the glass. This process has numerous drawbacks: first, that of requiring mobile, therefore more complex, blowing boxes, further the sheet is marked by the auxiliary supports and finally since the latter are integral with the base, the immobilization time of the latter is not reduced.